U.S.-backed Kurdish and Arab fighters have opened a new front against the Islamic State (IS) group in northern Syria.

Reports say the Syrian Democratic Forces (SDF) are pushing toward the IS-controlled town of Manbij, near the Turkish border, in an effort to deny the extremist group an area it uses to move supplies and foreign fighters across the border.

The Britain-based Syrian Observatory for Human Rights said on June 1 that the SDF troops seized control of 16 villages and were some 15 kilometers from Manbij.

Predawn air strikes on Manbij by the U.S.-led international coalition killed 15 civilians, it also said.

The clashes were "fierce and intense," said Nasser Haj Mansour, an adviser to the SDF.

Unnamed U.S. officials were quoted as saying thousands of forces were deployed in the operation, which could take weeks.

A small number of U.S. Special Operations Forces will support the push on the ground, acting as advisers, they said.